Since 1990s, progress of a light-emitting diode (LED) is remarkable whereby multicolor is progressing along with increase of output power. In particular, a white LED is expected to be a next generation light source to replace existing lamps such as a white lamp, a halogen lamp, and an HID lamp. In fact, an LED is valued in such characteristics as its long life, low power consumption, high heat stability, and low-voltage drive. Therefore, it is used in a display, a destination sign board, a car lamp, a signal light, an emergency light, a cell phone, a video camera, and so on. Such luminescent devices are usually manufactured by fixing an LED to a concave-shaped reflective material formed of a synthetic resin integrally molded with a lead frame and then by sealing it with a sealant such as an epoxy resin and a silicone resin.
As the reflective material for the LED, a polyamide resin is widely used nowadays. However, the reflectance decreases after a long-term usage due to the deterioration of the resin caused by increase of heat generation and optical intensity which is resulted from enhanced output power of the LED. Therefore, a measure to remedy this problem is desired.
To cope with this problem, for example, a reflective material made using a silicone resin is proposed in Patent Documents 1 to 3 and a reflective material made using an epoxy resin is proposed in Patent Document 4. But none of them has yet been used widely in practice. Especially when a silicone resin is used, problems are concerned in association with evaporation of a low molecular weight siloxane which may cause a contact malfunction and penetration of water vapor into the luminescent device which may cause damage to the light emitting element.
On the other hand, a reflective material made using a (meth)acrylate resin is disclosed in Patent Documents 5 and 6. In such reflective material, hollow particles are used as a filler to increase reflectance in the ultraviolet light region. It is disclosed that high reflectance in the visible light region can be achieved by laminating the material with a material which has high visible light reflectance.